How to assign xacro arguments to xacro macro parameters?

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I am initialising a macro in my xacro file as <xacro:macro name="sensor_d435" params="name:=realsense_d435_camera topics_ns:=realsense_d435 parent *origin">.

I have a xacro argument with the name robot_namespace in my launch file. I want to assign it to the topics_ns while initializing my macro, something like: topics_ns:=$(arg robot_namespace). When I do this and launch my original launch file, I get an error saying invalid expression: $(arg

Probably, it is seeing $(arg and robot_namespace) as 2 different and separate parameters. Any way how I can achieve what I want?

TIA